In order to produce hydrogen upon decomposition of ammonia in the presence of an ammonia decomposition catalyst, it is necessary to allow a reaction of the following equation (I) to proceed at a reaction temperature of 350° C. or higher.2NH3→3H2+N2 (endothermic reaction)  (I)
Though it is possible to allow the reaction of the equation (I) to proceed using a ruthenium based catalyst at a reaction temperature of 400° C. or higher, this reaction is an endothermic reaction, and therefore, in order to obtain a stable decomposition rate of ammonia, it is necessary to give heat to the reaction system.
A temperature drop in the case where 100% of ammonia is decomposed is about 900° C. In order to set a gas temperature in a downstream region of the catalyst layer at, for example, 350° C. or higher, it is necessary to set an inlet gas temperature at 1,250° C. or higher, and hence, such is not practical. Then, in order to suppress the gas temperature drop to be caused due to the endothermic reaction, heat was conventionally supplied from the outside. However, according to this method, since a rate of heat transfer is slower than the reaction rate, in order to obtain a sufficient rate of heat transfer, an area of heat-transfer surface must be made large, and it is difficult to achieve compactification of the apparatus.
In addition, a method of utilizing an exhaust gas of an ammonia engine as a heat source of heat supply from the outside may be considered. However, according to this method, in the case where the temperature of an engine exhaust gas is not higher than 350° C., this temperature is lower than a temperature at which the catalyst works. Therefore, there is involved such a drawback that the heat supply cannot be achieved, so that a prescribed amount of hydrogen cannot be produced.
As the heat source of heat supply, in addition to the supply from the outside, there is a method of generating heat by a reaction between ammonia and oxygen as shown in the following equation (II) and utilizing this heat.NH3+¾O2→½N2+3/2H2O (exothermic reaction)  (II)
If the reactions of the equations (1) and (2) are made to take place in the same reaction tube, it is possible to compensate heat for the endothermic reaction of the equation (I) by heat generated in the equation (II). In addition, by controlling the oxygen amount in the equation (II), the temperature of a catalyst layer can be controlled. For example, in the case where the temperature of a supply gas pre-heated by heat exchange of waste heat of the engine exhaust gas fluctuates, it becomes possible to stably produce hydrogen.
As a catalyst for ammonia oxidation, a platinum based catalyst is generally used. For example, Patent Document 1 proposes a multilayered ammonia oxidation catalyst composed of a refractory metal oxide, a layer of platinum disposed on this refractory metal oxide, and a layer of vanadia disposed on this platinum.
However, the working temperature of this catalyst is about 200° C., and if the temperature is not higher than this temperature, the oxidation reaction cannot be allowed to proceed, and hence, it is necessary to increase the gas temperature to about 200° C. by an electric heater or the like.
Patent Document 2 proposes an ammonia oxidation catalyst composed of an oxide of at least one element selected from cerium and praseodymium, an oxide of at least one element selected from non-variable valency rare earth elements including yttrium, and an oxide of cobalt. In addition, Patent Document 3 proposes an ammonia oxidation catalyst containing filaments substantially composed of platinum and rhodium and if desired, palladium, the filaments having a platinum coating. However, these patent documents have the same problem as that in Patent Document 1, too.